Generally, a mixer or a mixer circuit refers to a circuit for converting an input signal of one frequency band into a signal of a second desired frequency band. The mixer is widely used in a transmitter and a receiver of a communication system and other various fields.
For example, an RF mixer is an essential part of wireless communication systems. Modern wireless communication systems demand stringent dynamic range requirements. The dynamic range of a receiver is often limited by the first downconversion mixer. This forces many compromises between figures of merit such as conversion gain, linearity, dynamic, range, noise figure and port to port isolation of the mixer. Integrated mixers become more desirable than discrete ones for higher system integration with cost and space savings. In order to optimize the overall system performance, there exists a need to design improved mixers for integrated solutions.
Mixers perform frequency translation by multiplying two signals (and possibly their harmonics). Downconversion mixers employed in the receive path have two distinctly different inputs called the RF input and the LO (local oscillator) input. The RF input receives the signal to be downconverted, and the LO input receives the periodic waveform generated by the local oscillator. The performance parameters of downconversion mixers are the noise figure, the conversion gain, the input impedance, the 3rd order intercept point and the port-to-port isolation.
An example of a mixer is a mixer for use in a direct conversion receiver of a mobile communication system. The mixer mixes an input Radio Frequency (RF) signal with a signal from a Local Oscillator (LO) to output an Intermediate Frequency (IF) signal. The mixer is generally implemented with a Complementary Metal-Oxide Semiconductor (CMOS). The Receive Mixer down converts the high frequency of the signal received over the air to a lower frequency. The Noise Figure (NF) of this circuit is an important parameter and needs to be small. Next to the NF, the linearity is an important parameter. These two constraints require a considerable amount of power.
Passive mixers are often used in order to go to very small noise figures. In a passive mixer, there is no or little 1/f-noise. The passive mixer uses no dc-current through the switches but is very power hungry to make the interfacing with the preceding and following stage. In this sense, the passive mixer was not a good solution for a low power receiver such as a Bluetooth™ receiver.
As shown schematically in FIG. la, a double balanced active mixer comprises switches driven by a local oscillator (LO) that reverses the polarity of an RF input at the LO frequency. To get the highest performance from the mixer, the RF to IF path should be as linear as possible, and the switching time of the LO switch should be a minimum. The mixer performs frequency translations (conversion) by multiplication of an RF input signal with an LO signal. The balanced structure of FIG. la cancels any output at the RF input frequency since it will average to zero. It also cancels out any LO frequency component since the IF output is a differential signal and the LO appears as common mode. Such a mixer in reality has a noise problem (e.g., at the low end of the dynamic range) or intermodulation distortion (IMD) problem at the high end since the transconductors and loads such as resistors are non-linear and the switches are not ideal.
One form of double balanced active mixer is a Gilbert cell mixer. A disadvantage of the structure is the 1/f-noise produced by the switches which is determined by the amount of dc-current through the switches. The active Gilbert type cell mixer can be used in transconductance mode (current output); therefore, it is necessary to have a high output impedance.
The noise figure (NF) and the linearity behave inversely proportional to the dc-current through the switches. The higher the dc-current the better the linearity but the worse 1/f-noise. It is difficult or impossible to find a good trade-off. To improve the NF, the switch needs to be very fast and require a low threshold (i.e., a low Vgs-Vt), but this reduces the output impedance and degrades the overall receiver performance in terms of bandwidth, gain and linearity.
FIG. 1b is a circuit diagram of a known mixer according to US 2007/0126491. Referring to FIG. 1b, the known mixer includes Field Effect Transistors (FETs) M1-M6, a current bias source IBIAS, two load impedances RLOAD1, RLOAD2, and other FETs M7 and M8. The FETs M7 and M8 constitute a static current bleeding circuitry, and the other components constitute a so-called Gilbert cell mixer.
This mixer may be, for example, the mixer for a receiver (e.g., a mixer for use in downconverting from an RF frequency to an intermediate frequency IF). In this case, the gates of transistors M3 and M6 of the mixer are connected to outputs of an RF front end of a receiver (i.e., are coupled to the outputs RF+ and RF− of a linear amplifier that amplifies a received signal). The IF outputs IF+ and IF− of the mixer can be connected to the inputs of an IF unit. Oscillating signal outputs LO+ and LO+ of a local oscillator LO are connected to the gates of transistors M1 and M5, and M2 and M4, respectively. Thus, the mixer receives an RF signal as an input, mixes the input RF signal with a signal oscillated by the LO, and outputs a resulting IF signal.
A gate of FET M1 is connected to a first output LO+ of the LO, a drain is connected to a side of the first load RLOAD1 and a first input IF+ of an IF unit, and a source is connected to a source of the FET M2 and a drain of the FET M3. A gate of FET M2 is connected to a second output LO− of the LO and a gate of the FET M4, a drain is connected to a side of the second load RLOAD2 and a second input IF− of the IF unit, and a source is connected to the source of the FET M1 and the drain of the FET M3. A gate of FET M3 is connected to a first output RF+ of an RF unit, a drain is connected to the source of the FET M1 and the source of the FET M2, and a source is connected to a side of the current source IBIAS and a source of FET M6. The other side of the current source IBias is connected to ground.
A gate of FET M4 is connected to the second output LO− of the LO and the gate of the FET M2, a drain is connected to a side of the first load RLOAD1 and the first input IF+ of the IF unit, and a source is connected to a source of the FET M5 and a drain of the FET M6. A gate of FET M5 is connected to the first output LO+ of the LO, a drain is connected to a side of the second load RLOAD2 and the second input IF− of the IF unit, and a source is connected to a source of the FET M4 and a drain of the FET M6. A gate of FET M6 is connected to the second output RF− of the RF unit, a drain is connected to the source of the FET M4 and a source of the FET M5, and a source is connected to a side of the current source IBIAS and the source of FET M3. FETs M1-M6 may be implemented as P-type Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs).
One side of the first load RLOAD1 is connected to the first input IF+ of the IF unit and the drain of the FET M1, and the other side is connected to a supply voltage terminal VDD. One side of second load RLOAD2 is connected to the second input IF− of the IF unit and the drain of the FET M5, and the other side is connected to the supply voltage terminal VDD.
A gate of FET M7 is connected to a voltage source VBias, a source (or drain) is connected to the supply voltage terminal VDD, and a drain (or source) is connected to the sources of the FETs M1 and M2 and the drain of the FET M3. A gate of FET M8, is connected to another voltage source VBias, a source (or drain) is connected to the supply voltage VDD, and a drain (or source) is connected to the sources of the FETs M4 and M5 and the drain of the FET M6.
For the known mixer as shown in FIG. 1b, it is claimed that the flicker noise, also referred to as 1/f noise, can be effectively removed by the current bleeding circuitry formed by the FETs M7 and M8. In the mixer, flicker noise originates from the FETs M1 and M5. In other words, if a large amount of current flows through the sources of FETs M1 and M5, more flicker noise is generated. Thus, there is a need to ensure that only a small amount of current flows through the sources of the FETs M1 and M5. For a good conversion performance of the mixer, there is a need to ensure that a large amount of current flows through the FETs M3 and M6 (i.e., the FETs M3 and M6 from the sources of the FETs M1 and M5).
By using a current bleeding circuitry, the achievement of the conflicting requirements can be improved. The flicker requirement can be improved by ensuring that a small amount of current flows through the sources of the FETs M1 and M5, and the conversion requirement can be improved by ensuring that a large amount of current flows through the current bleeding circuitry (i.e., the FETs M7 and M8 in order to ensure that a large amount of current flows through the FETs M3 and M6). In practice, the conversion requirements and the noise requirements are not compatible.